Manufacturing manipulations require mechanical interaction with the environment or with the object being manipulated. Robotic manipulators are subject to interaction forces when they maneuver in a constrained workspace. Assembly of parts and deburring edges of workpieces are examples of constrained maneuvers wherein one is concerned not only with the position of the robot end point, but also the contact forces upon engagement with the workpiece. In constrained maneuvering, the interaction forces must be accommodated rather than resisted. If we define "compliancy" as a measure of the ability of such manipulators to react to interaction forces and torques, the objective is to assure compliant motion (passively or actively) toward the robot end point in the Cartesian coordinate system for manipulators that must maneuver in a constrained environment.
Robotic assembly is an example of a manufacturing manipulation that requires compliancy. The reason is that the parts typically are not perfectly aligned. To perform assembly of slightly misaligned parts, a compliant element is typically placed between the part and the robot to ease the insertion process. The remote center compliance (RCC) device can be attached to the end point of robot manipulators. This device provides a passive compliant interface between the robot and the part, and functions primarily as a filter to decrease the contact force between the part and the robot due to programming errors, vibration or isolation of the robot, and part fixturing errors. Such devices are considered passive because the elements that generate compliance are simply passive springs whose resistance is a function of their displacement. U.S. Pat. Nos. 4,537,557, 4,414,750 and 4,283,153 are representative of the prior art in this regard.
Although the RCC devices of the prior art have functioned reasonably well, they have limited controllability because their compliancy cannot be adjusted without at least some disassembly to change their springs. This of course involves interruption of the assembly operation.
Active end effectors have also been available heretofore. Hollis proposed such a device in an article entitled "A Planar XY Robotic Fine Positioning Device", IEEE Proceedings, July, 1985. That device incorporates electromagnetic positioning and computer control to obtain precise positioning, however, no provision is made for active compliance control.
A need has thus arisen for an active compliant end-effector whose compliancy is generated electronically so that it can be modulated in response to the task for more accuracy.